Yipp, one could say (if using varying %-tokens is officially suported) that "Number of images kept" in system settings just does not work ;] And it will fill every HD on earth after a period of time. I was really curious about how you would have solved this - damn :)
And yes, it´s not predictable the way it works currently. One (far from perfect) thing I could think of is to use some "reserved" suffix in the filename (+number as it is now) which get´s added by default. It could be parsed as already present autosave file regardless of the rest of the filename, but as consequence the autosave filelimit would affect every file group saved in a given directory.
But I would prefer this instead of having a directory like in the attachment (filelimit 3/3). And now that we get a %f directory (hooray! I wish we could use this in the standard max render output file/path), it even could make more sense like this when %f in the Corona autosave path will get default in future releases.
Good Luck